G-Protein Coupled Receptors
G-protein coupled receptors (GPCRs) constitute a major class of proteins responsible for transducing a signal within a cell. GPCRs have three structural domains: an amino terminal extracellular domain, a transmembrane domain containing seven transmembrane segments, three extracellular loops, and three intracellular loops, and a carboxy terminal intracellular domain. Upon binding of a ligand to an extracellular portion of a GPCR, a signal is transduced within the cell that results in a change in a biological or physiological property of the cell. GPCRs, along with G-proteins and effectors (intracellular enzymes and channels modulated by G-proteins), are the components of a modular signaling system that connects the state of intracellular second messengers to extracellular inputs.
GPCR genes and gene-products are potential causative agents of disease (Spiegel et al., J. Clin. Invest. 92:1119–1125 (1993); McKusick et al., J. Med. Genet. 30:1–26 (1993)). Specific defects in the rhodopsin gene and the V2 vasopressin receptor gene have been shown to cause various forms of retinitis pigmentosum (Nathans et al., Annu. Rev. Genet. 26:403–424(1992)), and nephrogenic diabetes insipidus (Holtzman et al., Hum. Mol. Genet. 2:1201–1204 (1993)). These receptors are of critical importance to both the central nervous system and peripheral physiological processes. Evolutionary analyses suggest that the ancestor of these proteins originally developed in concert with complex body plans and nervous systems.
The GPCR protein superfamily can be divided into five families: Family I, receptors typified by rhodopsin and the β2-purinergic receptor and currently represented by over 200 unique members (Dohlman et al., Annu. Rev. Biochem. 60:653–688 (1991)); Family II, the parathyroid hormone/calcitonin/secretin receptor family (Juppner et al., Science 254:1024–1026 (1991); Lin et al., Science 254:1022–1024 (1991)); Family III, the metabotropic glutamate receptor family (Nakanishi, Science 258 597:603 (1992)); Family IV, the cAMP receptor family, important in the chemotaxis and development of D. discoideum (Klein et al., Science 241:1467–1472 (1988)); and Family V, the fungal mating pheromone receptors such as STE2 (Kujan, Annu. Rev. Biochem. 61:1097–1129 (1992)).
There are also a small number of other proteins that present seven putative hydrophobic segments and appear to be unrelated to GPCRs; they have not been shown to couple to G-proteins. Drosophila expresses a photoreceptor-specific protein, bride of sevenless (boss), a seven-transmembrane-segment protein that has been extensively studied and does not show evidence of being a GPCR (Hart et al., Proc. Natl. Acad. Sci. USA 90:5047–5051 (1993)). The gene frizzled (fz) in Drosophila is also thought to be a protein with seven transmembrane segments. Like boss, fz has not been shown to couple to G-proteins (Vinson et al., Nature 338:263–264 (1989)).
G proteins represent a family of heterotrimeric proteins composed of α, β and γ subunits, that bind guanine nucleotides. These proteins are usually linked to cell surface receptors, e.g., receptors containing seven transmembrane segments. Following ligand binding to the GPCR, a conformational change is transmitted to the G protein, which causes the α-subunit to exchange a bound GDP molecule for a GTP molecule and to dissociate from the βγ-subunits. The GTP-bound form of the α-subunit typically functions as an effector-modulating moiety, leading to the production of second messengers, such as cAMP (e.g., by activation of adenyl cyclase), diacylglycerol or inositol phosphates. Greater than 20 different types of α-subunits are known in humans. These subunits associate with a smaller pool of β and γ subunits. Examples of mammalian G proteins include Gi, Go, Gq, Gs and Gt. G proteins are described extensively in Lodish et al., Molecular Cell Biology, (Scientific American Books Inc., New York, N.Y., 1995), the contents of which are incorporated herein by reference. GPCRs, G proteins and G protein-linked effector and second messenger systems have been reviewed in The G-Protein Linked Receptor Fact Book, Watson et al., eds., Academic Press (1994).
Aminergic GPCRs
One family of the GPCRS, Family II, contains receptors for acetylcholine, catecholamine, and indoleamine ligands (hereafter referred to as biogenic amines). The biogenic amine receptors (aminergic GPCRs) represent a large group of GPCRs that share a common evolutionary ancestor and which are present in both vertebrate (deuterostome), and invertebrate (protostome) lineages. This family of GPCRs includes, but is not limited to the 5-HT-like, the dopamine-like, the acetylcholine-like, the adrenaline-like and the melatonin-like GPCRs.
Dopamine Receptors
The understanding of the dopaminergic system relevance in brain function and disease developed several decades ago from three diverse observations following drug treatments. These were the observations that dopamine replacement therapy improved Parkinson's disease symptoms, depletion of dopamine and other catecholamines by reserpine caused depression and antipsychotic drugs blocked dopamine receptors. The finding that the dopamine receptor binding affinities of typical antipsychotic drugs correlate with their clinical potency led to the dopamine overactivity hypothesis of schizophrenia (Snyder, S. H., Am J Psychiatry 133, 197–202 (1976); Seeman, P. and Lee, T., Science 188, 1217–9 (1975)). Today, dopamine receptors are crucial targets in the pharmacological therapy of schizophrenia, Parkinson's disease, Tourette's syndrome, tardive dyskinesia and Huntington's disease. The dopaminergic system includes the nigrostriatal, mesocorticolimbic and tuberoinfindibular pathways. The nigrostriatal pathway is part of the striatal motor system and its degeneration leads to Parkinson's disease; the mesocorticolimbic pathway plays a key role in reinforcement and in emotional expression and is the desired site of action of antipsychotic drugs; the tuberoinfundibular pathways regulates prolactin secretion from the pituitary.
Dopamine receptors are members of the G protein coupled receptor superfamily, a large group proteins that share a seven helical membrane-spanning structure and transduce signals through coupling to heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins). Dopamine receptors are classified into subfamilies: D1-like (D1 and D5) and D2-like (D2, D3 and D4) based on their different ligand binding profiles, signal transduction properties, sequence homologies and genomic organizations (Civelli, O., Bunzow, J. R. and Grandy, D. K., Annu Rev Pharmacol Toxicol 33, 281–307 (1993)). The D1-like receptors, D1 and D5, stimulate cAMP synthesis through coupling with Gs-like proteins and their genes do not contain introns within their protein coding regions. On the other hand, the D2-like receptors, D2, D3 and D4, inhibit cAMP synthesis through their interaction with Gi-like proteins and share a similar genomic organization which includes introns within their protein coding regions.
Serotonin Receptors
Serotonin (5-Hydroxytryptamine; 5-HT) was first isolated from blood serum, where it was shown to promote vasoconstriction (Rapport, M. M., Green, A. A. and Page, I. H., J Biol Chem 176,1243–1251 (1948). Interest on a possible relationship between 5-HT and psychiatric disease was spurred by the observations that hallucinogens such as LSD and psilocybin inhibit the actions of 5-HT on smooth muscle preparations (Gaddum, J. H. and Hameed, K. A., Br J Pharmacol 9, 240–248 (1954)). This observation lead to the hypothesis that brain 5-HT activity might be altered in psychiatric disorders (Wooley, D. W. and Shaw, E., Proc Natl Acad Sci USA 40, 228–231 (1954); Gaddum, J. H. and Picarelli, Z. P., Br J Pharmacol 12, 323–328 (1957)). This hypothesis was strengthened by the introduction of tricyclic antidepressants and monoamine oxidase inhibitors for the treatment of major depression and the observation that those drugs affected noradrenaline and 5-HT metabolism. Today, drugs acting on the serotoninergic system have been proved to be effective in the pharmacotherapy of psychiatric diseases such as depression, schizophrenia, obsessive-compulsive disorder, panic disorder, generalized anxiety disorder and social phobia as well as migraine, vomiting induced by cancer chemotherapy and gastric motility disorders.
Serotonin receptors represent a very large and diverse family of neurotransmitter receptors. To date thirteen 5-HT receptor proteins coupled to G proteins plus one ligand-gated ion channel receptor (5-HT3) have been described in mammals. This receptor diversity is thought to reflect serotonin's ancient origin as a neurotransmitter and a hormone as well as the many different roles of 5-HT in mammals. The 5-HT receptors have been classified into seven subfamilies or groups according to their different ligand-binding affinity profiles, molecular structure and intracellular transduction mechanisms (Hoyer, D. et al., Pharmacol. Rev. 46, 157–203 (1994)).
Adrenergic GPCRs
The adrenergic receptors comprise one of the largest and most extensively characterized families within the G-protein coupled receptor “superfamily”. This superfamily includes not only adrenergic receptors, but also muscarinic, cholinergic, dopaminergic, serotonergic, and histaminergic receptors. Numerous peptide receptors include glucagon, somatostatin, and vasopressin receptors, as well as sensory receptors for vision (rhodopsin), taste, and olfaction, also belong to this growing family. Despite the diversity of signalling molecules, G-protein coupled receptors all possess a similar overall primary structure, characterized by 7 putative membrane-spanning .alpha. helices (Probst et al., 1992). In the most basic sense, the adrenergic receptors are the physiological sites of action of the catecholamines, epinephrine and norepinephrine. Adrenergic receptors were initially classified as either .alpha. or .beta. by Ahlquist, who demonstrated that the order of potency for a series of agonists to evoke a physiological response was distinctly different at the 2 receptor subtypes (Ahlquist, 1948). Functionally, .alpha. adrenergic receptors were shown to control vasoconstriction, pupil dilation and uterine inhibition, while .beta. adrenergic receptors were implicated in vasorelaxation, myocardial stimulation and bronchodilation (Regan et al., 1990). Eventually, pharmacologists realized that these responses resulted from activation of several distinct adrenergic receptor subtypes .beta. adrenergic receptors in the heart were defined as .beta..sub.1, while those in the lung and vasculature were termed .beta..sub.2 (Lands et al., 1967).
.alpha. Adrenergic receptors, meanwhile, were first classified based on their anatomical location, as either pre or post-synaptic (.alpha..sub.2 and alpha..sub. 1, respectively) (Langer et al., 1974). This classification scheme was confounded, however, by the presence of .alpha..sub.2 receptors in distinctly non-synaptic locations, such as platelets (Berthelsen and Pettinger, 1977). With the development of radioligand binding techniques, .alpha. adrenergic receptors could be distinguished pharmacologically based on their affinities for the antagonists prazosin or yohimbine (Stark, 1981). Definitive evidence for adrenergic receptor subtypes, however, awaited purification and molecular cloning of adrenergic receptor subtypes. In 1986, the genes for the hamster .beta..sub.2 (Dickson et al., 1986) and turkey .beta..sub.1 adrenergic receptors (Yarden et al., 1986) were cloned and sequenced. Hydropathy analysis revealed that these proteins contain 7 hydrophobic domains similar to rhodopsin, the receptor for light. Since that time the adrenergic receptor family has expanded to include 3 subtypes of .beta. receptors (Emorine et al., 1989), 3 subtypes of .alpha..sub.1 receptors (Schwinn et al., 1990), and 3 distinct types of .beta..sub.2 receptors (Lomasney et al., 1990).
The cloning, sequencing and expression of alpha receptor subtypes from animal tissues has led to the subclassification of the alpha 1 receptors into alpha 1d (formerly known as alpha 1a or 1a/1d), alpha 1b and alpha 1a (formerly known as alpha 1c) subtypes. Each alpha 1 receptor subtype exhibits its own pharmacologic and tissue specificities. The designation “alpha 1a” is the appellation recently approved by the IUPHAR Nomenclature Committee for the previously designated “alpha 1c” cloned subtype as outlined in the 1995 Receptor and Ion Channel Nomenclature Supplement (Watson and Girdlestone, 1995). The designation alpha la is used throughout this application to refer to this subtype. At the same time, the receptor formerly designated alpha 1a was renamed alpha 1d. The new nomenclature is used throughout this application. Stable cell lines expressing these alpha 1 receptor subtypes are referred to herein; however, these cell lines were deposited with the American Type Culture Collection (ATCC) under the old nomenclature. For a review of the classification of alpha 1 adrenoceptor subtypes, see, Martin C. Michel, et al., Naunyn-Schmiedeberg's Arch. Pharmacol. (1995) 352:1–10.
The differences in the alpha adrenergic receptor subtypes have relevance in pathophysiologic conditions. Benign prostatic hyperplasia, also known as benign prostatic hypertrophy or BPH, is an illness typically affecting men over fifty years of age, increasing in severity with increasing age. The symptoms of the condition include, but are not limited to, increased difficulty in urination and sexual dysfunction. These symptoms are induced by enlargement, or hyperplasia, of the prostate gland. As the prostate increases in size, it impinges on free-flow of fluids through the male urethra. Concommitantly, the increased noradrenergic innervation of the enlarged prostate leads to an increased adrenergic tone of the bladder neck and urethra, further restricting the flow of urine through the urethra.
The .alpha..sub.2 receptors appear to have diverged rather early from either .beta. or .alpha..sub.1 receptors. The .alpha..sub.2 receptors have been broken down into 3 molecularly distinct subtypes termed .alpha..sub.2 C2, .alpha..sub.2 C4, and .alpha..sub.2 C10 based on their chromosomal location. These subtypes appear to correspond to the respectively (Bylund et al., 1992). While all the receptors of the adrenergic type are recognized by epinephrine, they are pharmacologically distinct and are encoded by separate genes. These receptors are generally coupled to different second messenger pathways that are linked through G-proteins. Among the adrenergic receptors, .beta..sub. 1 and .beta..sub.2 receptors activate the adenylate cyclase, .alpha..sub.2 receptors inhibit adenylate cyclase and .alpha..sub.1 receptors activate phospholipase C pathways, stimulating breakdown of polyphosphoinositides (Chung, F. Z. et al., J. Biol. Chem., 263:4052 (1988)). .alpha..sub.1 and .alpha..sub.2 adrenergic receptors differ in their cell activity for drugs.
Issued US patent that disclose the utility of members of this family of proteins include, but are not limited to, U.S. Pat. No. 6,063,785 Phthalimido arylpiperazines useful in the treatment of benign prostatic hyperplasia; U.S. Pat. No. 6,060,492 Selective .beta.3 adrenergic agonists; U.S. Pat. No. 6,057,350 Alpha la adrenergic receptor antagonists; U.S. Pat. No. 6,046,192 Phenylethanolaminotetralincarboxamide derivatives; U.S. Pat. No. 6,046,183 Method of synergistic treatment for benign prostatic hyperplasia; U.S. Pat. No. 6,043,253 Fused piperidine substituted arylsulfonamides as .beta.3-agonists; U.S. Pat. No. 6,043,224 Compositions and methods for treatment of neurological disorders and neurodegenerative diseases; U.S. Pat. No. 6,037,354 Alpha 1a adrenergic receptor antagonists; U.S. Pat. No. 6,034,106 Oxadiazole benzenesulfonamides as selective .beta..sub.3 Agonist for the treatment of Diabetes and Obesity; U.S. Pat. No. 6,011,048 Thiazole benzenesulfonamides as .beta.3 agonists for treatment of diabetes and obesity; U.S. Pat. Nos. 6,008,361 5,994,506 Adrenergic receptor; U.S. Pat. No. 5,994,294 Nitrosated and nitrosylated .alpha.-adrenergic receptor antagonist compounds, compositions and their uses; U.S. Pat. No. 5,990,128 .alpha..sub.1C specific compounds to treat benign prostatic hyperplasia; U.S. Pat. No. 5,977,154 Selective .beta.3 adrenergic agonist; U.S. Pat. No. 5,977,115 Alpha 1a adrenergic receptor antagonists; U.S. Pat. No. 5,939,443 Selective .beta.3 adrenergic agonists; U.S. Pat. No. 5,932,538 Nitrosated and nitrosylated .alpha.-adrenergic receptor antagonist compounds, compositions and their uses; U.S. Pat. No. 5,922,722 Alpha 1a adrenergic receptor antagonists 26 U.S. Pat. Nos. 5,908,830 and 5,861,309 DNA endoding human alpha 1 adrenergic receptors.
Purinergic GPCRs
Purinoceptor P2Y1
P2 purinoceptors have been broadly classified as P2X receptors which are ATP-gated channels; P2Y receptors, a family of G protein-coupled receptors, and P2Z receptors, which mediate nonselective pores in mast cells. Numerous subtypes have been identified for each of the P2 receptor classes. P2Y receptors are characterized by their selective responsiveness towards ATP and its analogs. Some respond also to UTP. Based on the recommendation for nomenclature of P2 purinoceptors, the P2Y purinoceptors were numbered in the order of cloning. P2Y1, P2Y2 and P2Y3 have been cloned from a variety of species. P2Y1 responds to both ADP and ATP. Analysis of P2Y receptor subtype expression in human bone and 2 osteoblastic cell lines by RT-PCR showed that all known human P2Y receptor subtypes were expressed: P2Y1, P2Y2, P2Y4, P2Y6, and P2Y7 (Maier (Maier et al. 1997). In contrast, analysis of brain-derived cell lines suggested that a selective expression of P2Y receptor subtypes occurs in brain tissue.
Leon et al. generated P2Y1-null mice to define the physiologic role of the P2Y1 receptor. (J. Clin. Invest. 104: 1731–1737(1999)) These mice were viable with no apparent abnormalities affecting their development, survival, reproduction, or morphology of platelets, and the platelet count in these animals was identical to that of wildtype mice. However, platelets from P2Y 1-deficient mice were unable to aggregate in response to usual concentrations of ADP and displayed impaired aggregation to other agonists, while high concentrations of ADP induced platelet aggregation without shape change. In addition, ADP-induced inhibition of adenylyl cyclase still occurred, demonstrating the existence of an ADP receptor distinct from P2Y1. P2Y1-null mice had no spontaneous bleeding tendency but were resistant to thromboembolism induced by intravenous injection of ADP or collagen and adrenaline. Hence, the P2Y1 receptor plays an essential role in thrombotic states and represents a potential target for antithrombotic drugs. Somers et al. mapped the P2RY1 gene between flanking markers D3S1279 and D3S1280 at a position 173 to 174 cM from the most telomeric markers on the short arm of chromosome 3. (Genomics 44: 127–130 (1997)).
Purinoceptor P2Y2
The chloride ion secretory pathway that is defective in cystic fibrosis (CF) can be bypassed by an alternative pathway for chloride ion transport that is activated by extracellular nucleotides. Accordingly, the P2 receptor that mediates this effect is a therapeutic target for improving chloride secretion in CF patients. Parr et al. reported the sequence and functional expression of a cDNA cloned from human airway epithelial cells that encodes a protein with properties of a P2Y nucleotide receptor. (Proc. Nat. Acad. Sci. 91: 3275–3279(1994)) The human P2RY2 gene was mapped to chromosome 11q13.5–q14.1.
Purinoceptor P2RY4
The P2RY4 receptor appears to be activated specifically by UTP and UDP, but not by ATP and ADP. Activation of this uridine nucleotide receptor resulted in increased inositol phosphate formation and calcium mobilization. The UNR gene is located on chromosome Xq13.
Purinoceptor P2Y6
Somers et al. mapped the P2RY6 gene to 11q13.5, between polymorphic markers D11S1314 and D11S916, and P2RY2 maps within less than 4 cM of P2RY6. (Genomics 44: 127–130 (1997)) This was the first chromosomal clustering of this gene family to be described.
Adenine and uridine nucleotides, in addition to their well established role in intracellular energy metabolism, phosphorylation, and nucleic acid synthesis, also are important extracellular signaling molecules. P2Y metabotropic receptors are GPCRs that mediate the effects of extracellular nucleotides to regulate a wide variety of physiological processes. At least ten subfamilies of P2Y receptors have been identified. These receptor subfamilies differ greatly in their sequences and in their nucleotide agonist selectivities and efficacies.
It has been demonstrated that the P2Y1 receptors are strongly expressed in the brain, but the P2Y2, P2Y4 and P2Y6 receptors are also present. The localisation of one or more of these subtypes on neurons, on glia cells, on brain vasculature or on ventricle ependimal cells was found by in situ mRNA hybridisation and studies on those cells in culture. The P2Y1 receptors are prominent on neurons. The coupling of certain P2Y receptor subtypes to N-type Ca2+ channels or to particular K+ channels was also demonstrated.
It has also been demonstrated that several P2Y receptors mediate potent growth stimulatory effects on smooth muscle cells by stimulating intracellular pathways including Gq-proteins, protein kinase C and tyrosine phosphorylation, leading to increased immediate early gene expression, cell number, DNA and protein synthesis. It has been further demonstrated that P2Y regulation plays a nitogenic role in response to the development of artherosclerosis.
It has further been demonstrated that P2Y receptors play a critical role in cystic fibrosis. The volume and composition of the liquid that lines the airway surface is modulated by active transport of ions across the airway epithelium. This in turn is regulated both by autonomic agonists acting on basolateral receptors and by agonists acting on luminal receptors. Specifically, extracellular nucleotides present in the airway surface liquid act on luminal P2Y receptors to control both Cl− secretion and Na+ absorption. Since nucleotides are released in a regulated manner from airway epithelial cells, it is likely that their control over airway ion transport forms part of an autocrine regulatory system localised to the luminal surface of airway epithelia. In addition to this physiological role, P2Y receptor agonists have the potential to be of crucial benefit in the treatment of CF, a disorder of epithelial ion transport. The airways of people with CF have defective Cl− secretion and abnormally high rates of Na+ absorption. Since P2Y receptor agonists can regulate both these ion transport pathways they have the potential to pharmacologically bypass the ion transport defects in CF.
The present invention has a substantial similarity to purinergic receptor P2X-like 1 (purinoceptor P2X6). P2X-like purinergic receptor is an orphan receptor, and is known to be expressed in skeletal muscle. P2X6 belongs to the ATP-gated ion channel family. The rat P2X6 nucleotide sequence encodes a 379 amino acid protein that conserves all the structural features of previously cloned P2X receptors, including the two putative transmembrane domains predicted by hydrophobicity plots. In addition, P2XM, a subfamily of P2X-like purinergic receptor was localized to chromosomal band 22q11, where frequent loss of heterozygosity has been observed in rhabdoid tumors. This protein was implicated to play a significant role in the proliferation and/or differentiation of skeletal muscle cells and that its altered expression may be involved in the development of some sarcomas. For a review related to P2X-like purinergic receptor, see Spener et al., Mol. Cell. Biochem. 98 (1–2), 57–68 (1990); Peeters et al., Biochem. J. 276 (Pt 1), 203–207 (1991); Zanotti et al., J. Biol. Chem. 267 (26), 18541–18550 (1992); Troxler et al., Hum. Genet. 92 (6), 563–566 (1993); Huynh et al., Cancer Res. 55 (11), 2225–2231 (1995); Phelan et al., Genomics 34 (1), 63–68 (1996); Urano et al., Cancer Res 1997 August 1; 57(15): 3281–7; Soto et al., Biochem Biophys Res Commun 14; Jun. 14, 223(2): 456–60.
GPCRs, particularly members of the P2X-like purinergic receptor subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown GPCRs. The present invention advances the state of the art by providing a previously unidentified human GPCR.